Feature request for restructuredText formatted CHANGELOG

**Is your feature request related to a problem? Please describe.**
I currently use restructedText (rst) formatted `CHANGELOG.rst` but it seems release-please was unable to generate the rst-formatted files. I have opened the same feature request [here](https://github.com/google-github-actions/release-please-action/issues/636).

**Describe the solution you'd like**
Could you add the rst generator so that release-please can directly update rst-formatted `CHANGELOG.rst`?
**Describe alternatives you've considered**
For docs using Sphinx, I kept both md and rst files and used `mdinclude` directive provided by `sphinx_mdinclude` extension. But I am glad and it could be great if release-please can generate `CHANGELOG.rst` directly.

Thanks for your work!